1. Field of the Invention
The present invention relates to a plasma display panel (hereinafter referred to as PDP) which is used for displaying images on a television set, a computer or the like, and a process for producing such a plasma display panel, and also a thin film which is preferably used for covering a phosphor (or fluorescent) substance.
2. Description of Related Art
As is generally known in the art, a PDP is a self-emitting display which emits light by irradiating a phosphor substance with an ultraviolet (UV) ray generated by discharging a gas contained between a pair of substrates. For example, a surface discharge type PDP with a three electrode structure is shown in FIG. 11. (See Japanese Patent Kokai Publication No. H7-320645.) This PDP 80 has a front plate 60 and a rear plate 70 which are opposed to each other. The front plate 60 has a front substrate 61 and provided on an inner surface of the substrate 61 is an electrode pair 63 consisting of a pair of display electrodes “X” and “Y” adjacent and parallel to each other, and a dielectric layer 67 and a protective film 68 which cover the electrode pair. An outer surface of the front substrate 61 is a display surface 65. On the other hand, the rear plate 70 has a rear substrate 71 and provided on an inner surface of the substrate 71 are address electrodes “A” located in a direction perpendicular to the electrode pair 63, a plurality of ribs 74 projecting toward the front plate 60 while each address electrode “A” is between two adjacent ribs 74, and a phosphor layer 75 (75R, 75G, 75B) which covers a surface of the rear substrate 71 and the address electrodes “A” thereon between the ribs 74. A discharge space is formed between the front plate 60 and the rear plate 70 and filled with a discharge gas. Among these three electrodes “X”, “Y”, and “A”, the display electrodes “X” and “Y” delimit a surface discharge cell (a main discharge cell for display), and the display electrode “X” and the address electrode “A” delimit an address discharge cell for selecting an on or off state with respect to a unit emitting region “EU”. An ultraviolet ray generated by surface discharge between the display electrodes “X” and “Y” excites a phosphor substance in the phosphor layer 75, more specifically phosphor particles (not shown) to emit light.
In a case of a full color display, phosphor layers 75R, 75G, and 75B which have additive primary colors of red, green and blue are applied to constitute one pixel “EG” by the unit emitting regions “EU” of red, green, and blue. The phosphor layers 75R, 75G, and 75B are generally formed by applying a paste containing phosphor particles as a major component to the rear substrate 71 between the ribs with a screen printing method with respect to each color in sequence, followed by firing or baking of this obtained rear substrate 71.
As a red phosphor substance for the red phosphor layer 75R, a particulate material made of, for example, Y2O3: Eu having an average diameter of about 3 μm is used. As a green phosphor substance for the green phosphor layer 75G, a particulate material made of, for example, BaO.Al2O3:Mn having an average diameter of about 3 μm is used. As a blue phosphor substance for the blue phosphor layer 75B, a particulate material made of, for example, 3(Ba, Mg)O.8Al2O3:Eu having an average diameter of about 5 μm is used.
With respect to such a PDP, it is known to cover each phosphor particle with a film such as magnesium fluoride, calcium fluoride, barium fluoride or silicon oxide for various purposes. Procedures for forming the film are generally classified into two categories, a gas phase process (such as physical vapor deposition and chemical vapor deposition) and a liquid phase process (such as a solution method and a melt method).
For example, in an attempt to realize a higher luminescence and improve visibility and color reproducibility of a PDP, it is proposed to cover each phosphor particle with a thin film of a light-transmittance material having a refractive index lower than that of the phosphor particle, more specifically with a thin film of magnesium fluoride, silicon dioxide, alumina or the like. (See Japanese Patent Kokai Publication No. H7-320645.) This thin film is formed by a micro-encapsulation procedure such as evaporation, dipping, sputtering, and spraying methods.
For a purpose of realizing high emission efficiency of a PDP, it is proposed to cover each phosphor particle with an anti-reflection film, more specifically a film of magnesium fluoride, calcium fluoride, barium fluoride, silicon oxide or the like in order to decrease an amount of an ultraviolet ray which is lost by reflection at a surface of the phosphor particle. (See Japanese Patent Kokai Publication No. H10-228868.) The anti-reflection film is formed by chemical vapor deposition (CVD) or physical vapor deposition (PVD), or formed by preparing a suspension of phosphor particles in a dielectric melt and then pouring the suspension gently over a surface, e.g. a substrate.
In order to prevent degradation of luminance due to oxidation of the phosphor, it is also proposed to impart an oxidation resistance to the phosphor particle by coating the phosphor particle with magnesium fluoride or calcium fluoride. (See Japanese Patent Kokai Publication No. 2001-200249.) This coating film is formed by suspending phosphor particles in a solution and chemically depositing magnesium fluoride or calcium fluoride onto a surface of the phosphor particles.
For a higher luminance and a longer lifetime, it is proposed to cover each phosphor particle with a film of a dielectric material having a small loss due to absorbing an ultraviolet ray such as a fluoride of an alkaline metal or an alkaline-earth metal, e.g. a magnesium fluoride film, and to provide on this film an additional film having a higher secondary electron emission rate, e.g. a magnesium oxide film. (See Japanese Patent Kokai Publication No. H5-314912.) Methods for forming these films are not specified therein.
It is important for suppressing degradation of luminance which occurs during use of a PDP to prevent deterioration or degradation of the phosphor due to transformation thereof which is caused by irradiating the phosphor particles with a plasma (operative degradation). It is important for obtaining a higher luminance (or a higher emission efficiency) to supply a greater amount of an ultraviolet ray into the phosphor particle during use of the PDP. It is also important for higher luminance to prevent deterioration of the phosphor due to transformation which is caused by subjecting the phosphor particles to a gas containing moisture, oxygen and the like under a high temperature during sealing of the front plate and the rear plate in a process of manufacturing the PDP (process degradation) (for example, Eu2+ in the blue phosphor is transformed into Eu3+ by oxidation in presence of moisture). That is, the film for coating the phosphor particle needs to have a high UV transmittance and a high resistance (or protective property) to moisture, oxygen and plasma, especially to moisture.
In the prior art as described above, it is technically difficult and highly costly to form the film of fluoride, such as magnesium fluoride, by a liquid phase method.
On the other hand, when the fluoride film is formed by a general gas phase method the present inventors have found problems in that the film has a poor UV transmittance thereof and thus a luminance is lowered contrary to a purpose of obtaining a higher luminance. Results of tests by the inventors proved that when the fluoride film was formed by physical vapor deposition such as vacuum deposition and sputtering, and especially by sputtering, this obtained film had a brown color and a low UV transmittance. Conventional transparent films are not formed of a fluoride, such as a magnesium fluoride by a gas phase method.
In addition, since the fluoride is hygroscopic, the fluoride is not necessarily a sufficient material for coating a phosphor particle.